Embodiments of the present invention to be described can be applied to various kinds of Radio Access Technology (RAT), and to more than one RAT simultaneously. However, in order to introduce some of the concepts involved, some brief explanation will be made of relevant aspects of one such RAT, the Long-Term Evolution of 3GPP, usually abbreviated to LTE.
The basic system architecture in LTE is illustrated in FIG. 1. As can be seen, each mobile device (referred to in LTE as a UE) connects over a wireless link via a Uu interface to a base station (an eNB or eNodeB), which defines one or a number of cells for wireless communication.
Each eNB in turn is connected by a (usually) wired link using an S1 interface to higher-level or “core network” entities, including a Serving Gateway (S-GW) and a Mobility Management Entity (MME) for managing the system and sending control signalling to other nodes, particularly eNBs, in the network. The S1 interface can be subdivided into S1-U, the suffix -U denoting the user plane employed by the eNBs 11 for communicating user data to and from the S-GW; and S1-MME (sometimes called S1-C) for the control plane via which the eNBs exchange control messages with the MME.
The S-GW is responsible for packet forwarding of user data on the downlink to the UE and on the uplink. The S-GW provides a “mobility anchor” for the user plane during handovers of a UE from one eNB to another. It also manages and stores UE “contexts” which are the details of active connections with UEs.
The main function of the MME, as its name suggests, is to manage mobility of the UEs, and it is a signalling-only entity; in other words, user data packets do not pass through the MME. One eNB can have several S1-MME interfaces towards several MMEs. One function of the MME is to keep track of UEs as they move around the network: the MME maintains a register of UE identities and their locations. When there is downlink data intended for the idle-mode UE, the MME sends a Paging message including the UE's identity (device (ID) such as a Temporary Mobile Subscriber Identity (TMSI)).
The concept of “Tracking Area” (TA) is relevant to the invention to be described, and therefore will be briefly explained here. In a system such as LTE, the base stations (eNBs) form an overlapping of network of cells through which mobile devices may travel. As mentioned, the MME keeps track of UEs in the network. A tracking area is a group of cells in which a UE can move freely without having to update the MME with its location. Related to this, a Tracking Area List (TAL) is provided in LTE to allow the same cell to belong to more than one tracking area, allowing TAs to overlap and reducing signalling overhead. The UE refers to the TAL as it moves around the network, and only needs to update its location when it moves to a cell not in the TAL. The UE updates its location by sending to the network a location update message together with a device ID (e.g. its TMSI), allowing the MME to update the register.
Wireless communication systems are constructed by dividing the tasks to be performed among a plurality of layered protocols, each node or entity in the system being equipped to process data at various layers (or levels within a layer) in a protocol stack, with the protocols at corresponding layers notionally communicating with each other. Although ultimately all signalling in the system is carried by the lowest, physical layer, this hierarchical arrangement allows each layer to be considered independently.
FIG. 2 shows protocol layers for the control plane employed in LTE, by way of example. Each shaded box represents a different node in the system: UE, and eNodeB, and MME. Within each node, the protocols form a “protocol stack”. Thus, transmission of messages between nodes in a radio network, such as between the UEs and eNBs in FIG. 1, involves the use of multi-layer protocol stacks.
The protocols shown in FIG. 2 can be grouped in various ways. For example, the NAS (Non-Access Stratum), S1-AP (see below) and RRC (Radio Resource Control) protocols shown in FIG. 2 may be labelled Layer-3, the PDCP, MAC and RLC protocols as Layer-2, and the PHY (Physical layer) and L1 as Layer-1. Broadly speaking, on the transmission side Layer-3 is responsible for constructing message contents relating to mobility and session management for example, which are passed down to Layer-2 for further processing, including addition of headers etc. for transport purposes, and then passed down further to Layer-1 for transmission.
More particularly, Layer-2 includes a Packet Data Convergence Protocol (PDCP) sub-layer, a Radio Link Control (RLC) sub-layer, and a Media Access Control (MAC) sub-layer. The MAC layer forms S1 signalling messages or other data into data units (MAC PDUs) suitable for transmission over the radio network. These are received by the physical layer PHY, which provides the link from each network node to the radio resources of the network. On the reception side, starting at Layer-1 each layer decodes the header inserted in the corresponding transmission-side layer to allow reconstruction of a data unit, which is then passed up to the next higher layer.
Signalling messages are exchanged between the UE and eNodeB across the Uu interface, indicated by a vertical dashed line in FIG. 2. The S1 control plane interface (S1-MME) is defined on the link between the eNB and the MME.
The application layer signalling protocol is referred to as S1-AP (S1 Application Protocol). In FIG. 2, STCP stands for Stream Control Transmission Protocol. Put simply, SCTP provides a guaranteed connection over a connection-less packet network service such as Internet Protocol, IP. The SCTP layer ensures delivery of S1-AP application layer messages through SCTP association(s) established between two nodes. Application layer protocols submit their data to be transmitted in messages to the SCTP transport layer. SCTP places messages and control information into separate chunks (data chunks and control chunks), each identified by a chunk header. A message can be fragmented over a number of data chunks, but each data chunk contains data from only one user message. SCTP chunks are bundled into SCTP packets and each SCTP packet, which is submitted to the IP layer, consists of a packet header, SCTP control chunks when necessary, followed by SCTP data chunks when available.
The above mentioned SCTP “association” is a relationship between two SCTP endpoints. An endpoint is a set of transport addresses and a transport address consists of a network layer address and a port number. SCTP provides multi-streaming, in which several connections (streams) are bundled together into a single SCTP association, and each message sent over an SCTP association is assigned to a particular stream. All data within a stream is delivered in order with respect to other data in that stream, but data in different streams have no order constraints.
SCTP is an example of a “multi-homing” protocol. Multihoming can be used, for example, to increase the reliability of an IP-based network. In multi-homing, transparent fail-over is enabled between redundant network paths by using more than one IP address for one or both endpoints of a connection, as shown in FIG. 3. In FIG. 3, the endpoints of the SCTP connection are a Local Node (STCP Local Endpoint) and a Remote Node (SCTP Remote Endpoint). Each endpoint monitors the reachability of the secondary addresses of its peer so that it always knows which addresses are available for the failover. An SCTP identifies the endpoints such as IP-L 1, IP-L1 and IP-R1, IP-R2 shown in FIG. 3, but does not identify a service which may be provided by means of the SCTP association.
Although conventionally, a mobile device employs only one RAT at a time for its communication, mobile devices such as smartphones are increasingly capable of supporting more than one RAT simultaneously, for example LTE and Wi-Fi (the IEEE802.11xx group of standards). Moreover, several radio access networks (RANs) employing various RATs may be available in the same place, offering the possibility of multi-RAT communication to increase the overall bandwidth available to the UE. There may be some commonality of hardware between such radio access technologies. Thus for example the same base station unit may act as both an eNB in LTE and an access point (AP) for Wi-Fi communication. Such a unit is referred to henceforth as a BS/AP. Also, for convenience, the term “RAT” is also used to denote a wireless communication system employing a specific RAT. Thus, “multi-RAT communication” means communication via a plurality of wireless communication systems which involve the use of a plurality of different RATs.
Recently, the concept of “virtualisation”, which for some time now has been applied in wired computer systems, has received attention for use in mobile networks. This concept can be applied in various ways.
Firstly, and most commonly to date, mobile virtualisation can be used to provide hardware virtualisation on a mobile phone or connected wireless device. It enables multiple operating systems or virtual machines to run simultaneously on a mobile phone or connected wireless device, using a hypervisor to create secure separation between the underlying hardware and the software that runs on top of it. Such virtual machines are one example of “independent devices” as referred to below. The mobile industry became interested, in 2008, in using the benefits of virtualisation technology for mobile phones and other devices like tablets, netbooks and machine-to-machine (M2M) devices. One such example is using mobile virtualisation to create low-cost Android smartphones.
Semiconductor vendors such as ST-Ericsson have adopted mobile virtualisation as part of their low-cost Android platform strategy. Another use case for mobile virtualisation is in the enterprise market. Today, many consumers carry two mobile phones: one for business use and another for personal use. With mobile virtualisation, mobile phones can support multiple domains/operating systems on the same hardware, so that the enterprise IT department can securely manage one domain (in a virtual machine), and the mobile operator can separately manage the other domain (in a virtual machine). For example, VMware's Horizon Mobile allows employees to use a phone's native operating system for personal tasks, but then switch over to a virtual machine that runs a separate OS for business tasks. Thus, in effect, the virtual machines constitute independent devices in the same way as if the user carried multiple smartphones.
Secondly, mobile virtualisation is starting to be applied in the sense of the separation of a mobile device (in the form of a physical device supporting a particular service) and an identifier (e.g. associated with services an end user subscribes to). In other words, an end user may access a service from independent physical devices using the same identifier. Service continuity in this case, i.e. how to seamlessly switch a service from one device to another, becomes crucial. There are several solutions at application level. One such example is Amazon's Whispersync that allows synchronisation of books, videos, personal documents, and games across supported Kindle devices and apps. A user can pick up reading where they left off and view the bookmarks, highlights, and notes that were created on another device. Another example is Apple's Handoff, by which iPhone users running OS X Yosemite can seamlessly transition between workflows on their iPhone device and on a Mac laptop computer.
However, in this second sense, virtualisation is so far only available with specific applications and combinations of hardware. More general mechanisms are required before mobile virtualisation in this sense can be made available widely.